The following discussion of the background to the invention is intended to facilitate an understanding of the present invention. However, it should be appreciated that the discussion is not an acknowledgment or admission that any of the material referred to was published, known or part of the common general knowledge in any jurisdiction as at the priority date of the application.
Semiconductor wafer cutting or dicing is a cutting operation that is being employed to separate a semiconductor wafer, commonly but not necessarily always a silicon wafer, into individual smaller semiconductor chips. Cutting a wafer into individual chips may be accomplished by a number of methods, such as with the use of blade saws. However, this conventional method poses few problems. Firstly, the blades are prone to wear over time. The cutting quality is therefore affected over time with the use of the same blade. The operator has to predict the useful life-span of the blade and to replace a new one at the end of its useful life. The premature blade replacement results in high equipment cost. Secondly, the cutting conditions, such as the force exerted, the cutting speed, the cutting depth and the cutting angles, have to be precisely controlled to prevent any fracture or crack on the surface of the wafer. Additionally, the cutting process tends to create particles or chippings along the edges of the cutting path. As technology advances and the demand for miniature semiconductor devices increases, the need for thinner wafers also increases. This poses a serious challenge to convention blade-saw cutting method because direct mechanical cutting of a thin wafer inevitably introduces mechanical stress into the thin wafer and the thin wafer is more likely to experience fracture than a thicker wafer.
An alternative widely adopted cutting method to blade-saw cutting is laser cutting, which is a non-contact cutting process. Unlike the blade-saw cutting process, no mechanical stress is introduced into the wafer with the use of the non-contact laser cutting method. The use of different lasers, such as Q-switched 1,064 nm Nd:YAG lasers and their harmonics, UV lasers and with short pulse for a high optical absorption or a multi-photonic action with silicon for laser processing of a silicon wafer is well known to those of skilled in the art. Generally, laser beams focus on the target in a relatively short time and release energy simultaneously. Chemical bonds in the target material are broken by the photochemical action of the laser beam and cutting may be achieved by moving the scanning laser beam or the working platform to produce the desired shape. Due to the photochemical action of the laser beam to break the bonds in the silicon wafer, debris comprising deposits of silicon are produced on the surface of the wafer. The presence of the debris greatly reduces the quality of wafer. The debris has to be thoroughly removed in order to avoid failures in the subsequent packaging operations.
In an attempt to remove the debris produced by laser cutting, the use of assist gas comprising nitrogen, argon, air, oxygen or a mixture thereof has been proposed. For example, a first assist gas is supplied to a surface of a wafer during a first cutting phase and a second assist gas is subsequently supplied to the surface of the wafer during a second subsequent cutting phase. Silicon deposits, when formed, will be quickly removed by the assist gas thereby minimizing the deposition of silicon on the surface of the wafer.
In another attempt, the cutting of a silicon wafer is conducted in the presence of assist gas comprising sulfur hexafluoride (SF6). The laser beam is focused onto the silicon wafer surface at a power density above the ablation threshold of silicon so that the assist gas reacts with the silicon to form gaseous silicon tetrafluoride (SiF4). The deposition of silicon on wafer surface is therefore minimized. However, the use of assist gas translates to higher operating and material costs which render this approach less attractive.
The use of water jet-guided laser beam is another proposed method for cutting a silicon wafer. Water jet-guided laser is primarily based on guiding a laser beam inside a fine water jet. Because of the difference in the reflection coefficient of water and air, the laser beam is fully reflected at the water-jet surface, similar to the operation of an optical fiber. The advantage of this water jet-guided laser beam combination over the conventional laser cutting is that debris produced during the laser cutting is simultaneously removed from the cutting path due to the washout of the high pressurized water jet. This eliminates the need for assist gas. However, the presence of water is not desirable when cutting wafers with integrated circuits. Indeed, it is often required to avoid, if not minimize, the presence of water during laser cutting of a silicon wafer.
To-date, laser-cutting methods for separating a semiconductor wafer into individual semiconductor chips remain a satisfactory and convenient option compared to mechanical blade-saw cutting method. Despite this, existing laser-cutting methods pose a problem of debris contaminants depositing onto the wafer surface after cutting, which deposition eventually leads to the degradation of the resultant semiconductor device properties. Further, additional or post-treatment processes are often needed to remove the debris from the wafer surface, which may be complicated and time consuming.
The trend of electronic devices is moving towards higher speed, more integrated functions and compact volume. The conventional integrated circuit structure is inadequate to satisfy the ever-growing demand for higher performances. Better performing integrated circuits mandate thinner silicon substrate and the introduction of new materials or structures into the electronic devices. A thin silicon wafer is desired for several reasons. Thinner wafers facilitate the stacking of circuits, which directly leads to the increase in circuit density. By reducing the thickness of silicon bulk, the device is moving closer to the metal heat sink so that heat conducts away from the active area more effectively, which is critical for high-frequency operation. The mechanical flexibility of thin wafers is ideal for flexible systems, such as smart cards, chip-in-paper and contactless label. Today, the chip thickness is less than 250 μm and will be further reduced. Traditionally, a diamond saw blade is used to dice the wafer. Because of the contact nature of this technology, mechanical damages cannot be reduced without great sacrifice in the dicing speed. All the cutting conditions such as the force exerted, the cutting speed, the cutting depth and the cutting angles have to be well controlled properly to prevent any fracture or crack on the surface of the silicon wafer. Along with the reduced substrate thickness, new materials and structures are introduced to achieve the desired high performance, including inter-layer dielectric with a low dielectric constant, polyimide coating and copper interconnects. These new materials or structures have lower elastic modulus, lower mechanical strength and poorer adhesion between layers than the conventional silicon, which imposes serious difficulty in wafer dicing.
Therefore, it is desirable to provide a wafer cutting method and system that overcomes, or at least alleviates, the above problems.